1. Field of the Invention
This invention pertains to hydraulic drilling rigs and, more particularly, to hydraulic control systems for such drilling rigs.
2. Description of the Prior Art
Hydraulic drilling rigs have pronounced advantages over block and tackle drilling rigs. In the use of the hydraulic drilling rigs it is often necessary to maintain a constant upward pull on a drill pipe or tool within the well. In some instances this upward pull must be maintained to hold the pipe or tool stationary in the well. In other instances this constant pull must be maintained to provide a slight upward movement of the pipe or the tool in the well. While hydraulic drilling rigs enable these operations to be performed very easily, the primary hydraulic fluid flow is subject to considerable internal leakage which varies with the load of the pipe and temperature. This leakage causes the pipe to be unable to be held stationary or to be pulled upwardly at a uniform rate.